In recent years, starting with mobile phones, there has been a considerable demand for compact and high-speed electronic devices. This demand is satisfied by, e.g., increasing the speed of a clock signal in switching power supplies and the components of digital signal processing circuits. Accordingly, a high-frequency current increases in circuits, particularly in power supply circuits, so that an electromagnetic radiation increases and signal quality degrades in a considerable manner. Hence, there has been an increasingly vocal demand for higher performance of devices for decoupling power.
Since high-performance digital devices include high-speed circuits operating at high speed and low-speed circuits operating at low speed, the spectrum of electromagnetic waves leaking into a power distribution circuit is recently distributed over an extremely wide band of several hundreds kHz to several tens GHz. The current of a direct-current power supply of a semiconductor integrated circuit, which is relatively large and is mounted on a circuit board, reaches a high level exceeding several tens amperes. As shown in FIG. 1, the power distribution circuit is a circuit including a power supply circuit and a power distribution wire for supplying, to another circuit, power having been supplied from the power supply circuit.
Leaked electromagnetic waves propagate through the power distribution wire and other circuits to the power supply circuit and cause a failure on a circuit fed with power from the power supply circuit. In general, a number of signal wires are disposed around the power distribution wire of the board. Thus, leaked electromagnetic waves are readily coupled to a number of signal wires. The leaked electromagnetic waves having been coupled to the signal wires degrade signal quality and propagate as high-frequency current through the signal wires to an external signal cable of a digital device. Then, the external signal cable acts as an antenna to radiate unwanted electromagnetic waves at high level to the air.
Further, some of the leaked electromagnetic waves propagating through the power distribution wire pass through the power supply circuit and propagate through a commercial AC power supply line. The commercial AC power supply line acts as an antenna to radiate unwanted electromagnetic waves at high level to the air. Meanwhile, the leaked electromagnetic waves propagate through the power distribution wire while repeating reflection at some midpoint in the power distribution wire. Thus, some of the leaked electromagnetic waves also propagate through the signal wire and degrade the waveform of a signal.
As shown in FIG. 1, a drastic measure for solving the above problem is to prevent electromagnetic waves generated by circuit operations (e.g., a switching operation of a switching element) from leaking into the power distribution wire. In order to solve the problem, it is necessary to considerably reduce an impedance relative to a high frequency in all frequency bands included in electromagnetic waves when viewing the power distribution circuit from a circuit for generating the electromagnetic waves.
When the impedance of the power distribution circuit viewed from a transistor becomes closer and closer to 0, electromagnetic waves excited by the transistor reflect on the entrance of the power distribution wire and do not enter the power distribution circuit.
Conventionally, capacitors are used to reduce the impedances of power distribution wires. As components used for electric and electromagnetic devices, capacitors have a long history and various kinds have been put into practical use so far. At present, capacitors such as ceramic capacitors and solid electrolytic capacitors are developed. In the ceramic capacitors, a ceramic material deposited with a metal thin film is laminated in multiple layers. In the solid electrolytic capacitors, a porous compact of a metal such as tantalum and aluminum with a valve action is used as an anode, an oxide film is used as a dielectric, and a conducting polymer is used as a solid electrolyte.
As a solid electrolytic capacitor, the following is known: a solid electrolytic capacitor having polypyrrole or an alkyl substitute thereof as a solid electrolyte on a dielectric oxide film (e.g., Patent Document 1), or a solid electrolytic capacitor having polyaniline formed as a solid electrolyte on a dielectric oxide film and a method of manufacturing the same (e.g., Patent Document 2). As compared with conventional capacitors, conducting polymers higher in conductivity by two digits or more are used as solid electrolytes and thus these capacitors have a low equivalent series resistance. With the same capacitance, these capacitors can exert effects in a high frequency area which is larger by two digits or more than those of the conventional capacitors.
However, these capacitors have two-terminal structures for a charging/discharging function, so that these capacitors rapidly increase an impedance between terminals of a high frequency area exceeding 10 MHz. Thus, these capacitors have become unsuitable for the power distribution circuits of digital circuits. For this reason, multilayer feedthrough capacitors and capacitor arrays for connecting a number of small multilayer ceramic capacitor chips in parallel have been developed. However, it has been difficult to efficiently reduce an impedance in a high frequency area exceeding 10 MHz.
To handle higher frequencies, the configurations of filters have been also studied. For example, a surface-mount noise filter has been proposed which is composed of a meandering conductor and a ground conductor which are interposed between ceramic dielectric sheets (e.g., Patent Document 3). FIG. 2 is a sectional view showing the configuration of the surface-mount noise filter composed of the meandering conductor and the ground conductor which are interposed between the ceramic dielectric sheets.
As shown in FIG. 2, the conventional surface-mount filter is configured such that a first dielectric sheet 110, a second dielectric sheet 120, and a third dielectric sheet 130 are laminated. A first internal conductor 111, a meandering conductor 115, and a second internal conductor 112, which are used for transmitting a signal, are disposed on an interface between the first dielectric sheet 110 and the second dielectric sheet 120. A ground conductor 125 is formed on an interface between the second dielectric sheet 120 and the third dielectric sheet 130 so as to face the meandering conductor 115.
An end of the first internal conductor 111 is connected to a first signal electrode 151, an end of the second internal conductor 112 is connected to a second signal electrode 152, and the meandering conductor 115 is connected between the other ends of the first internal conductor 111 and the second internal conductor 112. With this configuration, it is possible to obtain a noise filter which is superior in noise absorbing property at high frequencies to a noise filter having a combination of a conventional inductance element and capacitance element.
In such a surface-mount filter, an electric signal inputted from one of the electrodes, e.g., the first signal electrode 115 is filtered and the filtered electric signal is outputted to the other (second signal electrode 152). However, in the surface-mount filter, a capacitance formed as a distributed constant is constituted of the ground conductor 125, the meandering conductor 115, and the dielectric sheets laminated between the conductors. Only with the distribution capacitance, it is difficult to efficiently reduce an impedance in a high frequency area exceeding 10 MHz. Hence, a part of the internal conductor is used as a meandering conductor, so that a capacitance and a series inductance are combined to enhance the effect of attenuating signals.
Further, in order to bring an impedance of a power distribution circuit closer and closer to 0 as viewed from a switching element such as a transistor, the following has been disclosed: a technique using a power distribution circuit as a low impedance line (e.g., Patent Document 4) and a technique for forming a low impedance line by using a technique for manufacturing a solid electrolytic capacitor (e.g., Patent Document 5).
A multilayer printed board disclosed in Patent Document 4 is configured as below:
The multilayer printed board is characterized in that ground layers are laminated respectively via insulating material layers on the upper and lower sides of a power supply layer having a power supply wire. Further, a signal layer having a signal wire is laminated via a second insulating material layer on one or both of the upper and lower ground layers. With this configuration, direct-current power can be supplied to circuit elements such as a semiconductor IC and an LSI mounted on a multilayer printed board as in the case where an independent power supply of a low impedance is provided separately. Moreover, it is possible to suppress the radiation of electromagnetic waves from electronic devices without interfering with high-speed and high-frequency operations of the circuit elements such as an IC and an LSI mounted on the printed board.
A transmission line component disclosed in Patent Document 5 is configured such that a cylindrical external conductor, which is made of a conductive material larger in diameter than an internal conductor, is coaxially disposed via a high permittivity insulating material so as to cover a surface of the internal conductor made of a conductive material, so that a coaxial line of an extremely low characteristic impedance is formed. The component is inserted in series between a power supply line of a printed circuit board and a power supply port of a high-speed radio frequency circuit element such as an LSI, so that direct-current power can be supplied as in the case where an independent power supply of a low impedance is provided separately for each high-speed radio frequency circuit element mounted on the printed circuit board. Additionally, high-frequency power supply current generated from the high-speed radio circuit element by a high-speed switching operation is caused to have a dielectric loss in the transmission line component, so that it is possible to suppress power supply coupling between a power supply line and a signal line and the flow of high-frequency power supply current from the power supply line of the printed circuit board to a power supply cable in an apparatus.
[Patent Document 1]
Japanese Patent Publication No. 4-56445 (Japanese Patent Laid-Open No. 60-37114)
[Patent Document 2]
Japanese Patent Laid-Open No. 3-35516
[Patent Document 3]
Japanese Patent Laid-Open No. 6-53046
[Patent Document 4]
Japanese Patent Laid-Open No. 2001-53449
[Patent Document 5]
Japanese Patent Laid-Open No. 2002-335107